When processing liquid slurries it is usually important that the slurries remain homogeneous to insure uniform treatment of the slurries. Where the solid matter tends to settle out of solution rapidly, it has been necessary to provide mechanical stirrers or other types of agitators, such as recirculating pumps, to prevent such settling. However, such devices have not proven to be effective where abrasive sludges are involved, and the problem is more acute where the settling tends to occur in pressurized tanks or columns.
Pressure filters employing tubular or hollow leaf type filter elements have many advantageous features relative to other types of liquid clarifiers. However, where highly viscous liquid filtrates are involved, the flow rates are necessarily low wherefor the retention times in the filters are high. Consequently, settling of the solids within the filters is in many cases a serious problem which results in non-uniform flow through the filter elements.
Another settling problem may occur when thick sludges or the like are moved through a processing system. Such systems commonly employ a pressurizable feed tank which is initially filled with a quantity of the sludge or liquid to be processed. The tank is pressurized with a suitable gas to force the sludge from the feed tank through the system. The retention time in the feed tank may be relatively long and relatively high pressures of several atmospheres may be used in such systems. Because of the highly abrasive nature of many sludges, and because of the high pressures involved, conventional pumps for recirculating the contents of the feed tanks leave much to be desired.
It is known in the prior art to recirculate a liquid in an open tank by means of a gas operated pump wherein a pressurized gas is fed into the lower end of a riser tube having its upper end open at a location above the liquid level in the tank. Where high tank pressures are involved, however, the quantities of gas required and the large pump sizes required to pump such large amounts of gas make such systems impractical.